The present invention relates to matrix printers and more particularly, the invention relates to print heads for and in matrix printers.
Conventional print heads of the type to which the invention pertains are usually constructed in that plural print elements, such as fairly long needles, are disposed to have their front ends or tips aligned vertically and in a mouth piece thereby defining a print column for any character. "Vertical" is to mean in relation to a horizontal extension of lines to be printed, horizontal in that sense being also the direction of relative advance of the print head across the printing surface during printing of one line.
Printing proper results from individually activating the elements such as pushing the needles forward by solenoid action, so that the tip of such an activated needle hits the ink ribbon and the paper. The head advances horizontally across the printing surface (e.g. paper) transversely to the direction of needle movement as well as to the vertical column, so that characters are composed from dots produced in steps through individual and selective activation and non-activation of the needles.
The German printed patent application No. 2,679,763 suggests a modification in that in addition to seven such print elements whose tips are arranged in a column, one provides a second column with additional but less than seven elements, i.e. only three. These three elements are located so that in many instances one can print in two columns at once, as these three elements are placed in positions frequently used in that they are horizontally in line, respectively, with three needle tips of the first column. The speed of printing can be increased accordingly. Of course, one will not always be able to print two columns at once, so that the speed cannot really be doubled. For this it appears that one would need a complete second column having also seven print elements. Moreover, the appearance of the characters is not quite satisfactory if one prints under such a limitation. In other words, this type of arrangement is a compromise between speed increase and appearance. Moreover, one still has to consider the maximum cycle rate with which a needle operating solenoid can be activated.
It can readily be seen that providing a full second column of print elements requires exactly doubling the number of print elements and of their drives. Consequently, the head will be larger and much heavier. More power is needed to drive such a head across the paper. A similar situation was actually encountered in a twelve-needle head as proposed in German printed patent application No. 2,729,495. The rather heavy head, however, was operated in the flying mode, i.e. it did not stop for printing any column. Clearly, a heavy head is more difficult to stop accurately.
Another problem encountered for and in multiple print heads relates to the space requirements. The print needles are required to have their front ends or tips placed rather closely, corresponding to the dimensions of the characters to be printed. The solenoids for each needle are comparatively large, and the needles must be guided in a curved path accordingly. Clearly, this clustering and needle guiding becomes more and more of a problem if the number of needles and solenoids clustered is increased. The French Pat. No. 1,531,666 shows a multiple column print head and attempted to solve the problem by laterally spacing the columns rather far from each other. Consequently, one column is still occupied with printing a character while another column begins already to print the next character. This kind of arrangement complicates the control of the print elements significantly, one has to call on two character patterns at the same time or in overlapping relation.
Another problem is the following. The mouth piece of the print head into which one runs the needles, have their bores rather closely spaced, particularly in the region of the front ends. The reason for this close spacing is, of course, the desire to print dots as close to one another as possible, so that the printed dots merge. The wall thickness between adjacent bores in the head may be as thin as 0.03 mm, which is about one mill. The thin barrier between two adjacent bores is subjected to significant wear when the needles move back and forth. This is particularly the case for those needles which are strongly curved because of the lateral offset of their activating solenoids. One has tried to avoid these trouble spots by not placing the print needles along a vertical line, but arranging them in two columns with a staggered arrangement of the ends. See also German printed patent application No. 2,119,416. Staggering the needles does indeed solve the problem of too weak a barrier between adjacent needle guiding bores; it does not improve print speed but actually slows it down.